Close to 100 million people worldwide acquire UTIs each year, and despite antibiotic treatment, about a quarter develop another such infection within six months. UTIs cause painful, burning urination and the frequent urge to urinate. In serious cases, the infection can spread to the kidneys and then the bloodstream, where it can become life-threatening.
Most UTIs are caused by E. coli that live harmlessly in the gut. However, when shed in the feces, the bacteria can spread to the opening of the urinary tract and up to the bladder, where they can cause problems. Conventional wisdom holds that UTIs recur frequently because bacterial populations from the gut are continually re-seeding the urinary tract with disease-causing bacteria.